This invention relates to microwave landing systems for aircraft and, more particularly, to a system including a set of antennas wherein data signals are transmitted in plurality of directions between frames of the transmission of guidance signals.
Microwave landing systems are widely used at airports throughout the world. The systems incorporate a set of antennas arranged in a cluster, and pointing in different directions for the radiation of guidance signals to aircraft at all directions of azimuth around the airport. A beam of electromagnetic energy is directed in the forward direction, looking down the runway at an incoming aircraft. This beam scans to and fro over a sector of azimuth to provide a highly accurate azimuth guidance signal for the aircraft. To both the left and right sides of the foregoing sector, if the sector coverage is less than .+-.40.degree., there are provided clearance sectors in which the aircraft is directed to the right or to the left for entering the central sector. At all other azimuth directions, the aircraft receives signals indicating that it is in a region which is out of coverage of the foregoing guidance signals, and accordingly, provides protection against false guidance.
A feature in the generation of the frames of signals which guide the aircraft is the use of time multiplexing of successive signals wherein the preamble of each time slot is used to identfy what signal is being received. Adequate time is presented in the pre-amble signal so as to give a receiver in the aircraft opportunity to lock onto and process the individual signals. At the inception of each frame of transmission of signals, there is transmitted a digital data message by modulating the carrier of the electromagnetic wave with differential phase shift keying (DPSK), the message identifying the nature of the transmission as well as providing a synchronization reference point for the subsequent signals to be transmitted in the frame of guidance signals. The remaining signals are simply scanning beam or clearance pulses of the carrier signal, which pulses emanate from specific ones of the antennas at specific instance of time. The data signal is transmitted through a forwardly directed antenna so as to provide the identification and synchronization at the inception of each signal frame for the incoming aircraft. Aircraft located at other directions around the airport may also receive the data signal if the signal is reflected from the terrain or building in other directions. Otherwise, the data signal is not received by the other aircraft. Only those aircraft receiving the data and synchronization signal can respond to the subsequent signals of the signal frame. All of these transmissions are in accordance with an international convention on aircraft operation, the governing organization establishing these standards being often referred to as the International Civil Aviation Organization (ICAO).
Differences exist in the nature of the equipment which may be available at various airports for implementing the microwave landing system (MLS). The basic system provides spacing between the signal frames to permit the transmission of an additional frame of signals, which additional frame is available from additional equipment provided by more advanced systems. Thus, in addition to the azimuth and elevation guidance available from the standard system, the advanced system includes further equipment for the transmission of highly accurate elevation guidance (flare guidance) as well as back-azimuth guidance which enable the reception of guidance in directions other than only the forward direction. Thus, in the advanced system, all of the frames are utilized, there being no empty signal frames. However, in the standard system, alternate frames are empty, and simply serve as dead time during which the system is inactive.
A problem arises in that the foregoing standard system provides for the transmission of data and clearance signals in only the certain forward sectors of the antenna transmission patterns. Preferably, such signals should also be transmitted in other directions so as to enable aircraft to more rapidly reach the path for landing on the runway. However, the use of onmidirectional antennas and the increased power required for the operation of such antennas would result in an undesirable high cost to the equipment.